Social justice is a matter of life and death. It affects the way people live, their consequent chance of illness, and their risk of premature death. We watch in wonder as life expectancy and good health continue to increase in parts of the world and in alarm as they fail to improve in others. Where systemic differences in health are judged to be avoidable by reasonable action, they are, quite simply, unfair. It is this that the Commission on Social Determinants of Health labels health inequity. Putting right these inequities – the huge and remediable difference in health between and within countries – is a matter of social justice. Reducing health inequities is for the Commission an ethical imperative. Social injustice is killing people on a grand scale.